The present invention relates to an electric power conversion apparatus for converting an AC power into a DC power and then reconverting the AC power into the DC power.
An electric power conversion apparatus of so-called 3-arm system has been known. An electric power conversion circuit used in this conventional power conversion apparatus comprises a first switch group having a plurality of series-connected switching circuits each having a switching element constructed of a transistor capable of controlling an electric conduction in one direction and a diode anti-parallelly connected with the switching element, second and third switch groups constructed in a way similar to the first switch group, and a capacitor, with the first switch group, the second switch group, the third switch group and the capacitor connected in parallel. This power conversion circuit consists of three switch groups and thus is called a 3-arm system with each switch group taken as one arm. This power conversion circuit full-wave rectifies an AC power by the first switch group and the second switch group to charge the capacitor and converts a DC voltage appearing across the capacitor into an AC voltage by the second switch group and the third switch group, thus reconverting the DC power into the AC power.
The conventional power conversion apparatus of 3-arm system has a problem that, since the second switch group is used for both power rectification and inversion, any shift in phase between an input voltage and an output voltage causes a large current to flow into the input side. A conventional practice to deal with this problem involves detecting abnormal conditions in frequency and phase at high speed and, when any anomaly is detected, interrupting the electric power conversion circuit from the electric power system by activating thyristor switches connected anti-parallel and arranged in the input side. However, the interrupting circuit using the thyristor switches is expensive and requires a drive circuit that outputs a higher voltage than that of a logic circuit, making it difficult to reduce the cost of the electric power conversion apparatus.
An object of the present invention is to provide an electric power conversion apparatus using a so-called 3-arm type power conversion circuit which can use a switching means with a low response speed as an interrupting circuit to prevent an overcurrent from flowing into the input side when a phase shift or difference occurs between the input voltage and the output voltage.
Another object of the present invention is to provide an electric power conversion apparatus which can protect the electric power conversion circuit from an overcurrent even when a switching means with a low response speed is used as an interrupting circuit.
Still another object of the present invention is to provide an inexpensive power conversion apparatus.
According to one aspect, the present invention provides an electric power conversion apparatus which comprises: an electric power conversion circuit having a first switch group, a second switch group, a third switch group, and a capacitor, all connected in parallel, the first switch group comprising a plurality of series-connected switching circuits, each having a switching element capable of controlling an electric conduction in one way and a diode connected anti-parallel to the switching element, the second and third switch groups being constructed in the same way as the first switch group; a reactor (first reactor) connected between an intermediate point in the first switch group and one of two input terminals of an AC power supply; an instantaneous current control system for outputting an instantaneous current control signal according to a DC voltage command value; an instantaneous voltage control system for outputting an instantaneous voltage control signal according to an output voltage command value; a drive signal generation circuit for receiving the instantaneous current control signal and the instantaneous voltage control signal and applying drive signals at predetermined timings to a plurality of the switching elements making up the first to third switch groups of the electric power conversion circuit to make the electric power conversion circuit perform a converter operation and an inverter operation; and an interrupting circuit connected between said one of the terminals of the AC power supply and the first reactor and adapted to enter an interrupting state to block the AC voltage of the AC power supply from being applied to the electric power conversion circuit when an overcurrent produced by a phase difference between the AC voltage of the AC power supply and the AC voltage converted by the electric power conversion circuit flows into the electric power conversion circuit.
More specifically, the electric power conversion apparatus of this invention further includes: a first connection circuit for electrically connecting an intermediate point in the second switch group to the other of the two input terminals of the AC power supply and to one of two AC output terminals; and a second connection circuit for electrically connecting an intermediate point in the third switch group to the other of the two AC output terminals through a second reactor.
For example, the drive signal generation circuit can be made to control conductions of a plurality of the switching elements making up the first and second switch groups to make the electric power conversion circuit perform the converter operation for charging the capacitor and to control conductions of a plurality of the switching elements making up the second and third switch groups to make the electric power conversion circuit perform the inverter operation for converting a DC voltage appearing across the capacitor into an AC voltage, thereby outputting the converted AC voltage from an intermediate point in the second switch group and from an intermediate point in the third switch group. It is also possible to charge the capacitor with only the first switch group.
In this invention, the interrupting circuit uses switching means with a slower response speed than that of semiconductor switching elements. Further, this invention includes a current detector for measuring an AC current flowing into the intermediate point in the first switch group and an overcurrent detection circuit for detecting an overcurrent based on an output of the current detector. The electric power conversion apparatus of this invention also includes a command input blocking circuit for blocking the instantaneous current control signal and the instantaneous voltage control signal from being applied to the drive signal generation circuit for as long as an overcurrent is detected by the overcurrent detection circuit.
As the phase difference between the input AC voltage and the output AC voltage of the electric power conversion circuit increases, an overcurrent flows in. The current flowing into the electric power conversion circuit is detected by the current detector. When the overcurrent detection circuit detects that the input current detected by the current detector is in excess of a predetermined value (there is an overcurrent), the command input blocking circuit blocks the instantaneous current control signal and the instantaneous voltage control signal from being applied to the drive signal generation circuit. This brings the switching elements making up the first to third switch groups of the electric power conversion circuit into a non-conducting state, thereby preventing an overcurrent from flowing through the switching elements and from destroying them. When the command input blocking circuit blocks the application of the instantaneous current control signal and the instantaneous voltage control signal to the drive signal generation circuit, the switching elements of the electric power conversion circuit become non-conducting, sharply reducing the overcurrent. As a result, the overcurrent detection circuit no longer detects the overcurrent and the command input blocking circuit allows the instantaneous current control signal and the instantaneous voltage control signal to be fed to the drive signal generation circuit. Hence, the electric power conversion circuit starts to perform the conversion operation again. If at this time the phase difference between the input AC voltage and the output AC voltage is still large, an overcurrent flows again. The overcurrent is detected by the overcurrent detection circuit, activating the command input blocking circuit in a manner described above to turn off the electric power conversion circuit. After this, until the phase difference between the input AC voltage and the output AC voltage becomes small enough, the above sequence of operation is repeated. If the overcurrent state continues to exist, the interrupting circuit with a low response speed is opened to completely interrupt the overcurrent. As described above, with this invention, until the interrupting circuit is opened, the command input blocking circuit intermittently interrupts an overcurrent, thereby protecting the switching elements used in the electric power conversion circuit from being destroyed by the overcurrent.
In protecting the switching elements by turning off the electric power conversion circuit, an output blocking circuit may be provided which blocks output signals of the drive signal generation circuit from being applied to the associated switching elements of the electric power conversion circuit. In that case, the electric power conversion circuit needs only to have a current detector for measuring an AC current flowing into the intermediate point in the first switch group, an overcurrent detection circuit for detecting an overcurrent based on the output of the current detector, and an output blocking circuit for blocking the drive signals from being output from the drive signal generation circuit for as long as an overcurrent is detected by the overcurrent detection circuit.
With this invention, even when the interrupting circuit, designed to block an overcurrent generated by a phase deviation between an input voltage and an output voltage from flowing into the input side, uses low-speed switches to lower the cost of the electric power conversion apparatus, the electric power conversion circuit can still be protected against an overcurrent.